The present invention generally relates to modems, and more particularly to a modem which has a speed notifying means and a data compression function so as to optimize the communication speed between modems.
In data communication using a telephone switching network which has a limit to the communication speed between modems, there is an increasing demand to further improve the communication speed due to the popular use of terminal equipment such as personal computers which make data communication via the telephone switching network.
As one method of meeting the above demand, there is a proposed modem which has a data compression function and modulating and demodulating functions. In other words, the proposed modem compresses the data transmitted from the terminal equipment and improves the effective throughput of the data communication between the terminal equipment to several times the communication speed which is determined by the modulation technique used between the modems. The value which is multiplied by the communication speed is referred to as a compression rate.
In the case of the communication between the terminal equipment using the modems described above, the communication speed between the terminal equipment and the modem can in principle be increased to a value which is a product of the compression rate and the speed determined by the modulation technique used between the modems or greater. As a result, it becomes possible to effectively utilize the effects of the data compression function of the modems, and the realization of such a system is awaited.
FIG.1 shows an example of a conventional modem together with associated terminal equipment and the like. In FIG.1, modems 10 and 20 are coupled via a transmission line, and a terminal equipment 1 is coupled to the modem 10 while a terminal equipment 5 is coupled to the modem 20. For the sake of convenience, only the construction of the modem 10 is shown and described because the construction of the modem 20 is the same as that of the modem 10.
The modem 10 includes a data transmission and reception part 30, a data compression and decompression part 40, a modem part 50, and a speed determining part 60. The data transmission and reception part 30 performs data transmission and reception between the modem 10 and the terminal equipment 1. The modem part 50 modulates a transmission signal with a predetermined modulation technique before transmitting the same to the modem 20, and demodulates a reception signal which is received from the modem 20. The data compression and decompression part 40 is provided between the data transmission and reception part 30 and the modem part 50. The data compression and decompression part 40 compresses the data from the data transmission and reception part 30 at a predetermined compression rate before supplying the same to the modem part 50, and decompresses the compressed data from the modem part 50 before supplying the same to the data transmission and reception part 30. The speed determining part 60 is notified of the speed of the reception signal from the modem part 50 and sets the communication speed between the data transmission and reception part 30 and the terminal equipment 1.
Hence, the modem 10 notifies the terminal equipment 1 of the communication speed which is determined by the modulation technique used between the modems 10 and 20, regardless of the data compression technique which is determined when the modems 10 and 20 are connected. In this case, when the communication speed of the terminal equipment 1 is assumed to be the communication speed which is notified from the modem 10, the terminal equipments 1 and 5 communicate at the communication speed which is determined by the modulation technique used between the modems 10 and 20, and it is impossible to bring out the effects obtainable based on the data compression function of the modem 10.
In order to bring out the effects of the data compression function of the modem 10 in FIG.1, the communication speed between the terminal equipment 1 and the modem 10 must be set in advance to a fixed value which is greater than or equal to a specific value. This specific value is a product of the communication speed between the modems 10 and 20 which is determined by the predicted modulation technique used between the modems 10 and 20 and the data compression rate which is determined by the predicted data compression technique used between the modems 10 and 20.
According to the conventional system shown in FIG.1, it is impossible to obtain the effects of the data compression even though the modems 10 and 20 have the data compression function if the terminal equipment 1 and 5 are designed to merely determine the communication speed between the respective modems 10 and 20 depending on the communication speed notified from the respective modems 10 and 20.
As described above, in order for the terminal equipment to effectively utilize the built-in data compression function of the modem when making the communication through the modem, the communication speed between the terminal equipment and the modem must be fixed in advance to a speed which takes into account the predicted effects of the data compression.
On the other hand, there is a proposed modem which notifies the terminal equipment of the communication speed which is established by the handshaking between the modems when the modems are connected, and thereafter changes the communication speed between the modem and the terminal equipment to the notified communication speed. For example, a modem employing the AT command proposed by Hayes Microcomputer Products, Inc. of the United States is such a proposed modem. By the appearance of such a proposed modem, there are now proposed systems in which the terminal equipment has the functions of changing the communication speed between the terminal equipment and the modem to the communication speed which is notified from the modem and thereafter making the data communication between the terminal equipment and the modem.
However, even when the terminal equipment which determines the communication speed between the terminal equipment and the modem depending on the communication speed notified from the modem is simply connected to the modem which has the built-in data compression function, it is impossible to bring out the effects of the data compression.